1. Field of the Invention
The present invention relates to novel water-swellable fibers, of which at least a part of the fiber outer layer portion is composed of a hydrophilic cross-linked polymer (hereinafter referred to as hydrogel), and which fibers have latent or visualized crimps and combine a high degree of water-swellability with excellent physical properties, and to a process for producing the same. The invention also relates to an industrially advantageous process for producing novel water-swellable fibers, of which at least a part of the fiber outer layer portion is hydrogelled, said process comprising putting a prescribed amount of a specific alkali on fibers composed of an acrylonitrile polymer (hereinafter abbreviated as AN polymer) and then heat-treating the fibers.
2. Description of the Prior Art
In recent years, polymers having a high degree of water-swellability have been used in a wide field of applications because of their particular functions. For example, attempts have been made on applications to diapers, sanitary products, etc. by utilizing the instantaneous high water-absorbing power of these polymers; or applications to soil-improving materials, instant sandbags, etc. by utilizing their high water-retentive capacity; or applications to soft contact lenses, artificial internal organs, surgical seaming materials, etc. on account of their intimate affinity to human tissues, and among these applications some are already entering on a practical stage.
As regards the water-swellable polymers (hydrogels) having possibilities of use in such a wide field of applications, cases are not few where it is preferable that they take the form of fibers to meet their use purposes, and several hydrogels in the form of fibers are known. However, although such existing natural or synthetic fibers have a certain extent of water-swellability, some have an extremely low water-swellability and others are water-soluble, and in any case they have been far from the category of water-swellable fibers which can absorb and retain an amount of water several to several hundred times their own weight, and moreover are water-insoluble. In Japanese Published patent application No. 42916/1977, there is disclosed a highly swellable fiber-shaped structure composed of acrylic fibers into which specific cross-linkages and a large amount of salt-form carboxyl groups have been introduced. However, since this fiber-shaped structure contains an extremely large amount of salt-form carboxyl groups introduced thereinto and is hydrogelled throughout the outer and inner layers of the fibers, it is provided, on one hand, with a high degree of water-swellability indeed, but on the other hand, it is so brittle that its physical properties are far from the conception of fibers. That is to say, it is the present situation that there are no water-swellable fibers having satisfactory properties, and thus the possession of a high degree of water-swellability while retaining fiber physical properties have been contradictory problems.
In such a situation, we have researched to overcome the above-mentioned fundamental difficulty and to obtain fibers having a high degree of water swellability while retaining the fiber physical properties, without using particular fibers as the starting material and without requiring a special cross-linking treatment step. As a result, we have found that, when an aqueous solution of a specific alkali-metal hydroxide is caused to act on fibers composed of an AN polymer (hereinafter abbreviated as AN fibers) in such a way that only the outer layer portion of the fibers is selectively rendered hydrophilic and cross-linked (hydrogelled), water-swellability can be advantageously given to the fibers without impairing the fiber physical properties. On the basis of this discovery, we have previously proposed an invention as Japanese patent application No. 46058/1978. However, when the fibers being treated are subjected to a hydrolytic treatment by immersion in an alkaline bath which is thought essential for causing the reaction to proceed uniformly, it is necessary to heat the whole bath, so that a large amount of energy is required. In addition, the alkaline bath is contaminated by by-products generated during the hydrolytic treatment, so that this makes it difficult to reuse the alkaline bath. Therefore, this process has required improvement from the standpoint of industrial practice.